Quantification of principal horizontal stresses inside a tunnel: An application of Fracture induced Electromagnetic Radiation (FEMR) technique in the Darjeeling-Sikkim Himalayas

Characterization of recent stresses, delineation of the locations of active faults/landslide prone slip planes and forecasting earthquakes are some of the emerging applications of the Fracture Induced Electromagnetic Radiation (FEMR) technique. It is believed that such radiation is polarized along the direction of maximum compression and proportional to the maximum applied shear stress. In this paper, we have demonstrated the use of the FEMR technique to calculate the magnitude and directions of principal horizontal stresses inside a tunnel of similar to 240 m length over a hilly road between Jorethang and Mayong near the village of Miyong in Sikkim, India by taking 11 "cross-sectional" measurements using a portable FEMR recording device called ANGEL-M. Although a reverse stress regime is obvious and established over the Himalayas, the results show a normal stress regime prevailing over this shallow, near-surface region of the tunnel and the direction of maximum shear stress changes along the tunnel length. The results were explained with the aid of a simple clay analogue model where normal reactivation of a thrust plane with changing dips was simulated. With this communication, we hope to encourage scientists and engineers to deploy the FEMR technique which is lesser known but quicker, non-destructive, cheaper, and more accessible than conventional methods in evaluating tunnel safety by calculating shallow stresses locally modified inside a tunnel.